1. Field of the Invention
The present invention relates to a technique for fabricating an optical fiber with a lensed endface and, more particularly, to a fiber lensing technique which utilizes wet chemical etching operations to form the desired shape of the lens.
2. Description of the Prior Art
In most optical communication systems, a lightwave signal must be coupled between an optical fiber and a device (e.g., a laser, LED photodiode, or another fiber). For most conventional arrangements, discrete lenses in bulk form are inserted in the light path in front of the fiber to couple the signal into or out of the core region of the fiber. Problems exist with these arrangements with respect to alignment of components, the size of the bulk optic components, signal loss, overall cost, etc. A partial solution has been developed in which a discrete lens element is attached (e.g., epoxied) directly to the endface of the fiber. This approach may reduce signal loss and the overall complexity of the arrangement. However, the operation involved in attaching the lens to the fiber is time-consuming and may unnecessarily increase cost. Further, over extended periods of time the adhesive may fail, and the lens may become misaligned or completely detach itself from the fiber.
One alternative to the epoxied lens technique rests upon the recognition that differently doped glass materials used to form fibers etch at different rates when exposed to common fluorine-based etchants such as hydrofluoric acid and buffered hydrofluoric acid. By controlling various etching parameters (e.g., strength, time, temperature), workers have discovered that, for example, the cladding can be preferentially etched (removed) with respect to the core, leaving an exposed, protuberance. According to P. Kayoun et al, Electronic Letters, 17(12), 400 (1981), such a "protuberance acts as a diffracting phase object with lens-like properties." (p.401, col. 1). The lens, however, had a depression in the center and exhibited a coupling efficiency of only 35%. See also P. Kayoun, U.S. Pat. No. 4,406,732. In both references 40% concentration of hydrofluoric acid was used to form the lens on MCVD single mode fibers having pure silica cores and boron-doped claddings. Kawachi et al, Electronic Letters, 18(2), 71 (1982) recognized that the depression prevented one from obtaining high quality microlenses. They describe the use buffered hydrofluoric acid to produce a mesa or "circular cone" (FIG. 1) on the end of a VAD single mode fiber having a Ge-doped core and a pure silica cladding. Fire polishing converted the mesa or cone into a "round-shaped" microlens (p.71, col.2), but no evidence of coupling efficiency or reproducibility was reported.
Etching/polishing techniques, such as those described above, naturally align the lens with the core, regardless of any deviation in core placement within the fiber. Moreover, the shape of the lens immediately after etching is not very important because, regardless of that shape, surface tension effects during subsequent fire polishing convert that shape into a sphere.
Although this chemical etching method of forming a fiber lens is a viable alternative, some problems remain, including its relatively low coupling efficiency (.eta. of approximately 40-50%) and the need to carefully control the fire-polishing operation. In addition, fire polishing is frequently incompatible with other aspects of the process (e.g., the presence of plastic coating or epoxy on the fiber; or the availability of only a short stub of fiber on which to operate).
More recently, H. Ghafoari-Sheraz, Optical and Quantum Electronics, 20, 493 (1988) used buffered hydrofluoric acid to construct a conical microlens on the end of an aluminum-coated VAD single mode fiber having a Ge-doped core and a pure silica cladding. A minimum coupling loss of about 3 dB was reported, which corresponds to a coupling efficiency of less than 50%. Although the author did not use fire polishing in these experiments, he explicitly allows for that possibility to form a round-shaped microlens.
Thus, a need remains in the prior art for a lensing technique which does not include critical manufacturing operations, does not require fire polishing, and is capable of reproducibly achieving coupling efficiencies greater than 50%.